Proteins
Dr. Mausumi Adhya
Associate Professor
Supreme Knowledge Foundation, West Bengal, India

Biological significance of proteins
Transportation and storage molecules
• Oxygen storage Protein: Myoglobin
• Oxygen transporting proteins: Hemoglobin
• Iron storage protein in liver: Ferritin
Repair and maintenance
• Vital in the maintenance of body tissues including development and repair.
Examples., hair, skin, eyes, muscles and organs all made from proteins.
Hormones
• Insulin, glucagon regulate blood sugar.

Antibody
• Antibody prevents infection, illness and disease.
Enzymes
• Enzymes serve as biological catalyst.
Energy
• After using proteins for body tissue maintenance and other necessary
functions the excess proteins are used for energy source.
Biological significance of proteins

Three-dimensional structure of proteins
 Protein has three dimensional structure having four levels
 Primary (1o)
 Secondary (2o)
 Tertiary (3o)
 Quaternary (4o)
Primary structure of proteins
• The order in which the amino acids are joined together by peptide bonds.
• Angiotensin II (𝐀𝐬𝐩—𝐀𝐫𝐠—𝐕𝐚𝐥—𝐓𝐲𝐫—𝐈𝐥𝐞—𝐇𝐢𝐬—𝐏𝐫𝐨—𝐏𝐡𝐞) involved in
normal blood pressure regulation in human. Any other order of amino acids in
this peptide would not function as angiotensin II.

Peptide/protein formation
When two amino acids condense, a dipeptide is formed.
The carboxylic acid group (– 𝐂𝐎𝐎𝐇) of one amino acid reacts with the
amine group (– 𝑵𝑯𝟐) of a second amino acid.
A water molecule is lost and an amide functional group (−𝑪𝑶 − 𝑵𝑯 −) is
formed between the two amino acids.
This amide bond is called peptide bond. Structures are always written from
𝐍–terminus to 𝐂–terminus.

R1 C OH
O
+ H N R2
H
H2O
R1 C N
O H
R2
N
H2 CH C OH
R1 O
N
H2 CH C OH
R2 O
+
H2O
N
H2 CH C
R1 O
NH CH C OH
R2 O
N
H2 CH C
R2 O
NH CH C OH
R1 O
+
C N
O H
Amide Bond
Amide Bond formation

N
H2 CH2 COOH N
H2 CH COOH
CH3
+
H2O
N
H2 CH2 CO NH CH COOH
CH3
N
H2 CH CO
CH3
NH CH2 COOH
+
Gly Ala
Ala Gly
Gly Ala
Peptide bond formation

The two most common secondary structures
• alpha-helix (–helix)
• beta-pleated sheet (–pleated sheet)
• Structure stabilized by hydrogen bonding along the protein backbone
between amino acids close together in sequence.
Secondary structure of proteins

Alpha-helix (–helix)
 Coiled structure, much like the coil of a
telephone cord.
 Right-handed coil.
 Coil stabilized by hydrogen bonds between the
carbonyl oxygen (-C=O) of one amino acid and
the amine hydrogen atom (-N—H) of another
amino acid located four amino acids from
earlier in the primary structure.

Beta-pleated sheet (–pleated sheet)
• Parallel –pleated sheet
• Antiparallel –pleated sheet
 The -pleated sheet is an extended structure in which segments of the protein chain
align to form a zigzag structure.
 Strands called beta strands are held together through hydrogen bonding interactions
between the backbones.
 The side chains of a -pleated sheet extend above and below the sheet.

 Both angles are in same side.
 Both sheets start from N terminal
site of protein.
Parallel –pleated sheet

• Angles are opposite to each other.
• One sheet starts from N terminal and other
sheets starts from c terminal of the protein.
Antiparallel –pleated sheet

 The interactions of the side chains within the secondary structure lead to the
tertiary structure of proteins.
 The tertiary structure is the final specific geometric shape that a protein assumes.
Tertiary Structure
Interactions in the tertiary structure
Noncovalent
• Hydrophobic interactions (methyle-methyle / methyl- phenyl / phenyl-
phenyl)
• Electrostatic or hydrophilic interactions
• H-bonding
Covalent
Disulfide bond (𝑺 − 𝐒) formed from thiol groups (– 𝐒𝐇) of two cysteine molecules

S S H
H
S S H
H
S S H
H

C
H3 NH3 O
C
O
H
+
C
O
H O H NH NH2
C
H3 CH3
disulphide bond
hydrogen
bond
hydrophobic
interaction
salt bridge
polypeptide chain
tertiary structure of protein

Quaternary structure
 The proteins having subunits consist of quaternary structure.
 Two or more polypeptide chains interacting to form a biologically active
protein.
 Interactions in the quaternary structure same like tertiary structure.
 Examples
• Hemoglobin consists of four polypeptide chains or subunits has quaternary
structure.
• Myoglobin is a monomer and hence it has no quaternary structure.

S S H
H
H
H
H
H
H
H
S S H
H
C
H
3
CH
3
C
C
C
C
C
C
C
C
H
NH3
O
H
+

chain
chain
hydrophobic
interaction
H-bonding
ionic interaction
disulphide
linkage
Quaternary structure of protein

Classification of proteins according to shape (structure) and solubility
Fibrous proteins
• Long rod like structure and have high helical content.
• Not soluble in water.
• Example, keratins (found in hair, nails, and the scales of reptiles), elastin,
and collagen .
Globular proteins
• More or less spherical in nature.
• Highly soluble in water.
• Example, hemoglobin, myoglobin, albumin, insulin, many enzyme.

Classification of proteins according to shape (structure) and solubility
Membrane proteins
 Embedded in lipid bilayer
 Not soluble in aqueous solution.
 Rhodopsin is an example of a membrane protein.

Summary of levels of structure and stabilizing forces in proteins
Level of
structure
Forces stabilizing structure
Primary (𝟏𝟎
) Peptides bonds
Secondary (𝟐𝟎
) Hydrogen bonding along the protein backbone between
amino acids close together in sequence
Tertiary (𝟑𝟎
) London forces, hydrogen bonding, dipole-dipole and ion-
dipole interactions, salt bridges and disulfide bonds
between amino acids
Quaternary (𝟒𝟎
) London forces, hydrogen bonding, dipole-dipole and ion-
dipole interactions, salt bridges and disulfide bonds
between subunits

Hemoglobin A: Val-His-Leu-Thr-Pro-Glu-Glu-Lys-
Hemoglobin S: Val-His-Leu-Thr-Pro-Val-Glu-Lys-
Importance of Protein Structure